Does 20mm crushed rock mean every stone is 20mm?

No. It means the largest stones are around 20mm, with a graded spread of smaller sizes down to fines included. That mix of sizes is what lets it compact into a firm layer. A single-sized 20mm aggregate, with the fines screened out, is a different product.

Should I use 20mm or 40mm road base?

Use 40mm for deep lifts and bulk filling where you want to build depth quickly, and 20mm for thinner layers, the top of a base or a finer finished surface. As a guide, the layer should be at least two to three times the nominal stone size thick.

What is the difference between nominal size and class?

Class is the quality grade and pavement position (Class 2, 3, 4). Nominal size is the largest stone in the mix (20mm, 40mm). You specify both, for example Class 3 20mm, to fully describe the product.

What Does 20mm or 40mm Mean in Crushed Rock?

Quick answer

The nominal size is the largest stone in the mix. "20mm crushed rock" means the biggest pieces are around 20mm, with everything smaller down to fines included.

It is separate from the class. 40mm builds deep layers faster; 20mm gives a finer, tighter finish. Bigger nominal sizes suit thick lifts and bulk work, smaller sizes suit thin layers and a neater surface.

What the number refers to

Nominal size is the top of the grading. A 20mm product is engineered so the largest stones are about 20mm, with a continuous spread of smaller sizes beneath them down to dust. A few stones may sit slightly above the nominal figure; it is a description of the mix, not a hard cut-off on every particle.

Key point: a 20mm crushed rock is not all 20mm stones. It is a graded blend with 20mm as the largest. That spread of sizes is what lets it compact. A single-sized 20mm aggregate is a different product with the fines removed.

Size versus class

These two numbers describe different things and you need both to order correctly.

01Class (Class 2, 3, 4) is the quality grade and where it sits in the pavement.

02Nominal size (20mm, 40mm) is the largest stone in the mix.

03So "Class 3 40mm" is sub-base quality rock with stones up to 40mm. "Class 2 20mm" is base quality with a 20mm top size.

When to use 20mm vs 40mm

	20mm	40mm
Layer thickness	Thinner layers, finer finish	Thicker lifts, deep build-up
Surface	Tighter, smoother to finish	Coarser surface
Speed of build	Slower for deep fills	Builds depth quickly
Typical use	Driveways, top of base, paths	Deep sub-base, bulk fill, capping

Rule of thumb: the layer should be at least two to three times the nominal size thick, so 40mm suits deeper layers and 20mm suits shallower ones.

A quick rule

Match the stone size to the layer depth. Going deep and filling fast, choose 40mm. Building a thinner, neater layer or the top of a base, choose 20mm. If you are unsure what the spec wants, send the drawing to us with your quote and we will confirm the size and class together.

What does the nominal size (20mm, 40mm) mean?

What the number refers to

Size versus class

When to use 20mm vs 40mm

A quick rule

Right size, right class, one delivery.

What the number refers to

Size versus class

When to use 20mm vs 40mm

A quick rule

Common questions

Related guides

The products

Right size, right class, one delivery.